Semiconductor device fabrication



July 6, 1965 T. W. COOPER ETAL SEMICONDUGIOR DEVICE FABRICATION Filed 001A 27. 1960 5o Z/zaf A 2 Sheets-Sheet 1 July 6, 1965 T. w. COOPER ETAL SEMICONDUCTOR DEVICE FABRICATION 2 Sheets-Sheet 2 Filed 0G13. 27, 1960 iza- /z A *WM United States Patent O 3,193,413 SEMHCONDUCTDR DEVICE FABRCATEN Theodore W. Cooper, Santa Ana, and Preston V. Cheney,

Costa Mesa, Calif., assignors by mesme assignments, to

Fairchild Camera and Instrument Corporation, Syosset,

NX., a corporation of Delaware Filed Oct. 27, 1960, Ser. No. 71,858 1 Claim. (Cl. 14S-189) This invention relates to semiconductor device fabrication, and more particularly to an improvement in photochemical masking and subsequent fabrication of semiconductor devices.

In semiconductor device fabrication, it is desirable to produce sharply defined silicon oxide areas one-the surface of a silicon crystal. Such areas may be used as masks for subsequent operations, such as in-difusion of boron into monoxidized `areas for doping purposes, or for formation of insulating oxide layers. Suitable silicon oxide layers may be formed by oxidizing the surface of silicon semiconductor crystals, or by decomposing chemicals, such as .silanes, on a semiconductor such as silicon or germanium. It is presumed that such films are silicon Idioxide, and its properties appear to be those of silicon dioxide; whether silicon monoxide is also present in such films `is not known. Y

Defined areas of such silicon oxide films have been produced by masking portions of the films with special grease, solid masks such as rubber, and the like, and -subsequently etching or dissolving away the exposed portion of the film to leave an oxide film conforming to the shape of the mask. Such film masking techniques are difficult -t-o use, require great operator skill, are sources of contamination, and do not lend themselves to production of complicated patterns. Rubber masks particularly require pressure on the film surface, hence on the crystal, and both distort the mask and impose stresses on the crystal which tends to cause excessive breakage.

In the 4production of silicon semiconductor devices, such as PNP transistors, `for example, silicon slices may be formed from `P-type semiconductor crystals, indiffused with arsensic to form N-type surface regions, and then indiffused with boron to form a surface P-type region. I-t is is desirable in such transistors to mask a portion of the iN-type surface region during diffusion of the P-type impurity, boron, so that the final structure will have both N and yP type regions at the exposed surface for attachment of leads. A layer -of silicon oxide may be used as a mask for the boron diffusion process, but the formation of such masks has heretofore presented many problems of contamination, registry ofthe mask, sharpness of edges of the silicon oxide film, special operator skills and the like. Selective etching of silicon oxide in the presence of silicon to produce a silicon oxide film mask has been very difficult, and has produced uneven quality and deterioration or removal of the desired silicon oxide mask.

yPhotosensitive polymerizable materials have been proposed `as masks to produce patterns on silicon oxide films, so that a subsequent removal of unprotected silicon oxide will leave a silicon oxide pattern corresponding to that of the photosensitive polymerizable material. Such materials have produced lincomplete protection-to subsequent silicon oxide selective etchants. The edges of the silicon oxide films so produced are uneven and poorly defined, and often the entire .silicon oxide film is attacked or completely removed due to physical separation of the protective mask. lIt thus appears that the adherence and sealing quality of such materials is unreliable for use with silicon oxide films.

It is an object and advantage of this invention to avoid the foregoing difficulties and to produce an effective and sharply defined silicon oxide area, or mask. For im- 3,193,418 Patented July 6, 1965 ICC .purity diffusion into silicon, over 0.3 micron of silicon oxide film is generally required; on germanium over 0.15 micron is generally effective.

In accordance with the present invention, an improved masking technique covers an effective silicon oxide film or layer on a semiconductor crystal with an adherent metal film which is resistant to a .silicon oxide etch (HF), and then covers the metal with a photosensitive polymerizable (PSP) material, exposes and develops the PSP material to polymerize the exposed portion and remove from the metal lrn the unpolymerized portion thereof, selectively removes the metal from the oxide in the areas where the unpolymerized PSP material wasl removed, and then selectively removes the silicon oxide from the areas where the unpolymerized PSP material was removed. The .polymerized PSP material and the remaining portion of the metal film will ordinarily then be removed prior to subsequent oper-ations such as indiffusion lof boron or phosphorus linto the portions of the silicon crystal from which the silicon -oxide film was removed. The metal used is preferably germanium but may be chromium, nickel, cadmium, or alloys thereof such as nickel-chromiumk alloys. 'Germanium and vchromium are generally preferred, although chromium is preferred when processing germanium semiconduct-ors. Germanium is more readily and easily evaporated than chromium, its thermal expansion coefficient more nearly matches that of silicon, and it is more easily preferential-ly etched in the presence of silicon oxide and silicon. Both germanium and chromium have satisfactory adherence to silicon oxide and to the PSP material; have thermal expansion coefficients close enough to that of silicon to be useable as a lm thereon; and are sufficiently resistant to the hydroouric (HF) acid etchant used to .preferentially remove silicon oxide from the silicon crystal. It has been observed that the germanium to silicon oxide bond is stronger than its bond to silicon, and thus is a very good bond for providing additional resistance to the silicon oxide Afilm during etching thereof to form a pattern or mask of the -silicon oxide on the silicon.

It is thus a further object and advantage of this invention to produce a superior silicon oxide pattern, or mask, on a semiconductor, such as silicon, base materialutilizing photochemical techniques together with an intermediate metal film to protect the silicon oxide film d-uring the preferential etching thereof to produce the desired pattern in the silicon oxide film. The above and other objects and advantages of this invention will be apparent from the balance of this specification, disclosing the preferred embodiment of this invention, and in the accompanying drawings and claim forming a part thereof, wherein:

FiGS. l to l0 illustrate a sequential process Ifor pro-A ducing a silicon oxide pattern on a silicon semiconductor crystal according to this invention; and

FIGS. l1 to 21'illustrate a further stepwise process utilizing the silicon oxide film so produced. in 4the production of silicon transistor crystal elements.

According to this invention as illustrated in t-he drawings, FIG. 1 shows a P-type silicon semiconductor crystal 30 upon which a silicon oxide mask is to be formed for selective diffusion of boron as a dopant, or electrical conductivity type determining impurity. Various cleaning and degreasing steps which are well known in semiconductor operations are omitted herein for clarity of presentation. The Vcrystal 3f) is subjected to an atmosphere of As2O3 in argon -at about 1200" C., to indiffuse arsenic and form an N-type region 31' on the crystal surface. The balance of the crystal 30 will be a P-type region 32. The above atmosphere will simultaneously grow a silicon oxide film 33 on the crystal, hereinafter often referred to as an lSiOz film, which is preferably enhanced- The arsensic indiifusionY of example, tov produceV .an N-type region of 4 to 6VV -rnicrons depth'and a surface SiOZ film 33 vof 0.3 to`1.0

micron thickness, preferably 0.4 to 0.6 micron, as shown in FIG. 2. 'Proportions have been exaggerated in the drawing for illustrative purposes.

A germanium film 34 is next formed on the SiO2 film by evaporation of germanium ina vacuum furnace. -Fiams of from 0.15 to 9.0 microns have been used successfully, but 0.4 to 0.6 micron gives effective coverage and uniformity. This film is preferably formed by laying germanium on a tungsten filament, and heatingthe filament ,toV evaporate the germanium lin a vacuum of about 5 10-5 mm. I-Ig to deposit the germanium on an exposed surface of the crystal at about 550 C. having the Si02 film thereon. FIG. 3 shows theY resulting crystal 30 having an N-type region 31, a P-type region 32 for the 'balance of the crystal, an SiOz film 33 on the surface of the N-type region, and a germanium metal film 34 on the oxide film.

A film 35 of photosensitive polymerizable (PSP )V material, such as polyvinyl alcohol, or a product well Vknown on the market and sold Aunder the trade name of Kodak Photo Resist by Eastman Kodak Company and believed to be a resinous ester of maleic anhydride and alkoxy hydroxy acetophenone, is next formed on the surface of the germanium, as shown in PEG. Y4. The germanium is preferably lightly etched in a 4% hydrochloric acid etch or applied in any suitable Way. The film of PSP the unexposed, hence unpolymerized, PSP material,

leaving areas, for example, stripes, of polymerized PSP material 36 as shownv in FIG. V6. The crystal is then baked at 70 C. to further polymerize Vand hardenthe film portions 36.

Exposed germanium between stripes of PSP material 36 is next etched and removed, in an etchant such as hydrogen peroxide and oxalic acid, toexpose SiO2 between the stripes of PSP material 3b as shown in FIG-7. This etchant evolves relatively little gas when used below 40 C., and appears to have no substantial deleterious effec on the PSP material film. f

The exposed Si02 is next etched and removed by a hydrouoric acid etch which selectively removes SiOZ in the presence of silicon, germanium, and PSP material. FIG. 8 shows the resulting structure with Valternate ex-V posed areas of silicon crystal and stripes of layered PSP material, germanium and Si02; n

The PSP material 36 is next removed by softening with an appropriate solvent, such as methyl ethyl ketone, acetone, or trichloroethylene,'and subsequent brushing, to expose the germanium film 34 in stripes as shown in PlG.

Y V9. The crystal 30 is then subjected to a germanium sol-v vent etch, such as hydrogen peroxide an oxalic acid, to expose the SiOZ stripes on the crystal surface as shown in FIG. 10.

The production of the SiOZ pattern, in stripes as illustrated in FIG. 10, Without rough edges or undercutting of the Si02 and without loss of the Voxide film in the body of theV stripe, is the main objective of this invention. The SiO2 pattern-thus produced may be used in several alternative Ways.. By way vof example, the further processingV sistor fabrication uses the above Si02 stripes as a mask yin a boron diffusion processV wherein the crystal surface isV exposed to a boron containing gas such as boron oxide to diffuse bo-ron into the crystal between the oxide stripes and convert'the adjacent crystal region 3S to P conductivity type, as shown in FIG. 11.

An additional oxide film 33A as shown in FIG. 12 is next grown over the entire crystalface, as vby exposure to a 30 C. dew point argon atmosphere at about 900 C. In some cases, especially when the oxideV film 33 was relatively thick, it may be'preferable to remove the oxide stripes of that film beforegrowing the new film 33A.

A film 39 of chromium as shown in FIG. 13 is then vapor depositedon the'oxide film 33A on crystal, by vaporizing chromium from Va tungsten filament in a vacuum furnace. The chromium film 39 is then covered with aV film 40. of PSP material as shown in FiG. 14.

The PSP material of film 40 is next exposed to ultraviolet light through a photographic film mask positioned to expose and polymerize squares 41 bridging the junctionS;betWeen the P-regions 3S and the N-regions 31 at the surface ofthe crystal, underlying they several films, as shown in FIG. 15. TheV PSP material of film 40 is then rinsed with a solvent for the unploymerized PSP material, such as methyl ethyl ketone, to remove unpolymerized PSP material and leave the pattern of square areas 41 of polymerized PSP material bridging the P and N regions of the crystal surface. The material of the squares 41 is then baked and hardened as with the previous film stripes` 36. Y

The chromium'film now uncovered is Vnext removed by a70 to 80 C., 50% hydrochloric acid in water etchant to expose the Si02 film not Vcovered by the squares 41, as shown in FIG. 16. g

The SiOZ film is then etched in HF to expose the silicon crystal not covered by the squares 41', asshown in FIG. 17, and the silicon crystal is then etched, with (1:122) solution of nitric acid, hydroiiuoric acid and acetic acid, for example, to remove silicon crystal material down into. the lower P-region, leaving mesas of crystal under each square 41, as shown in FIG. 18.

VThe square layers of polymerized PSP material and chromium, are next Aremoved in successive etching operations utilizing a PSP material solvent such as trichloroethylene, with brushing of the softened PSP material, and a chromium solvent such as 37 to 39% HC1 at 50 to C. The resultingrstructure, shown in FlG. 19, is next provided with an ohmic contact on the vreverse crystal face, as by fusing alminurn '42 thereto as shown in FG. 20 forA subsequent attachment of a collector. The crystal is then sliced to separate out crystal elements '43 as shown in FIG. 2l, each having a mesa structure thereon protected by an oxide film, for subsequent device fabrication by removal of the oxide film, attachment of `leads and encapsulation.

l under the PSP material during-selective removal of the Si02 film,rillustrated as a germanium film, thus provides sharper delineation of the oxide mask or film areas under the polymerized PSP material, and the use thereof has sharply reducedffinishcd device rejects in silicon transistor'manu'facture, due to etching of the -SiOzffilm 33, from as highas 40% Without the germanium film to less than 1/z% with the germanium intermediate film. As will be apparent, a'chromium metal film could be used in the place of the germanium film 34 herein illustrated, with suitable adjustmentsV in the process such as the use of chromium solvents, or etchers, for chromium removal, or cadmium, nickel, or alloys of the above metals may be used as intermediate metal films as taught herein. The metal film requires a sufficient resistance to hydrofluoric acid used to etchr the silicon oxide film to protect the oxide thereunder, and a good adherence to the oxide and to the PSP material. A metal etch should also be utilized for metal removal which evolves a minimum of gas, and accordingly has little tendency to disturb the PSP material iilm during the etching process.

Although the example illustrated herein is a process for producing a PNP transistor, the process is Well suited for production of diodes or other semiconductor junction devices. To produce diodes an original N-type crystal 30 may be used, and the resulting structure will correspond to that shown in the drawings except that the N-type region 31 Would extend through the region 32 in the drawings. The advantages of the process as described, particularly in producing well dened and complicated silicon oxide masking films and the resultant precisely formed P-N junctions when such lms are used as diffusion masks, will accrue to a wide variety of junction semiconductor devices.

We claim:

In the process of fabricating a semiconductor device by the diffusion of a conductivity-type-determining impurity into selected portions of a semiconductor body through openings in a silicon oxide coating on said semiconductor body, the improvement comprising:

(1) forming a silicon oxide coating on a surface of said silicon semiconductor body by thermal oxidation;

(2) depositing from a vapor on said oxide coating an adherent film of metal selected from the class comprising germanium, chromium, cadirnum, nickel and alloys thereof, said film having a thermal coeiicient matched to said body;

(3) forming a lm of photo-sensitive polymerizable material on said metal film;

(4) exposing, developing, and selectively removing undeveloped portions of said photo-sensitive polymerizable material to expose selected areas of said metal lrn to expose selected areas of said oxide film;

(5) removing said exposed areas of said metal film to expose selected areas of said oxide film;

(6) removing the entire thickness of the oxide lm underlying said exposed areas of said oxide film to expose a selected surface portion of said semiconductor body;

(7) and thereafter subjecting said exposed surface portion of said semiconductor body to an atmosphere containing said conductivity-type-determining impurity to thereby diiuse said impurity into selected portions only of said semiconductor body.

References Cited bythe Examiner UNITED STATES PATENTS 1,922,434 8/ 33 Gundlach.

2,215,128 9/40 Meulendyke 95-5.7 2,666,008 1/54 Enslein et al 41-43 X 2,802,760 8/57 Derick et al 14S-1.5 2,878,147 3/59 Beale 14S-1.5 2,900,580 8/59 Beck 317-101 2,911,539 11/59 Tanenbaum 14S-1.5 X 2,946,683 7/60 Mellan et al 117-34 X 2,967,766 1/ 61 Witmore et al 156-14 X 2,977,227 3/ 61 Demaria 156-14 X 2,977,252 3/61 Causse et al 117-34 X 2,981,877 4/61 Noyce 307-101 3,012,920 12/61 Christensen et al. 156-11 3,025,589 3/62 Hoerni 29-25.3 3,064,167 11/62 Hoerni 317-235 OTHER REFERENCES Maneld: Using Thin Films in Microminiaturization, Electronic Design, January 21, 1959, vol. 7, No. 2, pp. 38, 39, and 40.

Swiggett et al.: The Economics of Printed Wiring, Tele-Tech and Electronic Industries, December 1953, pp. 78, 79 and 80.

JOHN W. HUCKERT, Primary Examiner.

MARCUS U. LYON, DAVID RECK, GEORGE N.

WESTBY, DAVID J. GALVIN, Examiners. 

